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The change only handles files moved/renamed withing the same
BUILD.gn file. If files are moved beyond the same BUILD.gn file,
the affected BUILD.gn files should be fixed manually.

BUG=404616
TEST=1) rename files with git mv
     2) run tools/git/mass-rename.sh
     3) confirm that BUILD.gn is updated as expected

Review URL: https://codereview.chromium.org/894973002

Cr-Commit-Position: refs/heads/master@{#314945}